Transgenic avians (e.g., transgenic chickens, quails or turkeys) are a desirable expression system for obtaining exogenous recombinant proteins for use in pharmaceutical or other commercial applications that require large amounts of protein supply. A hen can lay up to 330 eggs per year, each containing 6.5 grams of protein. About 3.5 grams of the total protein is from egg white, of which 90% is accounted for by seven different proteins; the ovalbumin alone accounts for 2 grams of egg white protein (about 50% of the egg white protein). Currently, the average exogenous gene product derived from oviduct specific expression of a transgene and recovered from the egg white is known to be about 5-10 mg per egg. Advantages of exogenous protein production in chicken eggs include short generation times and prolific rates of reproduction via artificial insemination. Various proteins have been expressed in eggs of transgenic chickens. See, e.g., U.S. Pat. No. 6,730,822 and U.S. Publication No. 2006/0015960.
Many exogenous therapeutic proteins (e.g., recombinant human proteins such as cytokines (e.g., erythropoietin, granulocyte colony-stimulating factor (GC-SF), interferons, and granulocyte-macrophage colony-stimulating factor (GM-CSF)), antibodies, and various human lysosomal enzymes) are of interest to the pharmaceutical industry. The therapeutic proteins can readily be obtained in significant quantities from, for example, egg white of transgenic chickens. Traditional methods of isolating exogenous proteins from the egg white, however, often rely on use of immunoaffinity procedures or other procedures only suitable for small scale production (e.g., involving total egg white volume of 5 L at most). For a large-scale protein production, such a procedure is not practical based on costs, labor, and time.